trigger ContactDuplicateTrigger on Contact (before insert) {
   for (Contact c : Trigger.new){
      
      Contact[] contacts= [select id from Contact where FirstName = :c.FirstName and LastName = :c.LastName and Email = :c.Email];
      
      if (contacts.size() > 0) {
          c.LastName.addError('Contact cannot be created - Contact already exists');
      }    
   }
}